During light engine load conditions, such as after a driver releases an accelerator pedal, it may be desirable to provide driveline braking to control vehicle speed. If insufficient driveline braking is provided, the vehicle may coast longer than is desired, thereby making the driver uncomfortable. On the other hand, if the driveline braking is greater than is desired, the vehicle will decelerate faster than is desired and fuel economy may be reduced. One way to respond to release of an accelerator pedal is to cease fuel flow to engine cylinders while the engine cylinders continue to induct air. The engine pumps air through the cylinder and the pumping work provides a negative driveline torque to slow the vehicle. However, pumping air through the cylinder also upsets an amount of oxygen stored in a catalyst downstream of the engine. Further, the pumping of air by the engine may provide more or less negative torque to the driveline so that the vehicle may not decelerate at a desired rate.
The inventor herein has recognized the above-mentioned disadvantages and has developed an engine operating method, comprising: operating a first actual total number of cylinders not combusting air and fuel with cylinder poppet valves remaining in closed positions over an engine cycle in response to a desired engine brake torque; and operating a second actual total number of cylinders not combusting air and fuel with cylinder poppet valves that open and close over the engine cycle in response to the desired engine brake torque.
By adjusting an actual total number of cylinders not combusting air and fuel to a first number in response to a desired engine brake torque and adjusting the actual total number of cylinders not combusting air and fuel to a second number in response to the desired engine brake torque, it may be possible to provide the technical result of being able to operate in two different modes where different driveline braking may be desired. For example, it may be desirable to operate an engine with a first actual total number of deactivated cylinders to provide a desired driveline brake torque in a hill descent mode and a second actual total number of deactivated cylinders to provide the desired driveline torque in a non-hill descent mode. The different number of deactivated cylinders may provide a closer match to the desired engine torque in the different vehicle modes and the engine may be better prepared for exiting one mode and entering a second mode when the actual total number of deactivated cylinders may be varied.
The present description may provide several advantages. For example, the approach may improve engine braking control. Further, the approach may provide the benefits of cylinder deactivation such as lower engine pumping work. Additionally, the approach may be applied in conjunction with transmission shifting control to further improve vehicle deceleration control in various vehicle driving modes.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.